The situation in Houston is already devastating, as floodwaters continue to rise. The National Hurricane Service on Sunday morning updated its rain predictions, adding 10 more inches, from a possible 40 inches in places to 50 inches. Shortly thereafter, the National Weather Service tweeted that Harvey — now a tropical storm — is an “unprecedented event.”

A scientist with FEMA also tweeted that “[t]his could be the most rainfall ever observed from a tropical cyclone in United States history.”

In 2001, tropical storm Allison killed 23 people in Texas and caused billions of dollars in damage. That storm, just 16 years ago, was considered a 500-year flood event. Since then, Houston has continued to develop, paving more land and stopping up ways for water to flow. Even the city’s bayous are lined with cement.

As of noon on Sunday, most of the city’s flood channels exceeded capacity.

CREDIT: Harris County

On Sunday, President Donald Trump tweeted that Harvey is also a once-in-500-year flood. “We have an all out effort going, and going well!” he said.

Thousands of people are in need of rescue, and more rain is on the way, as Harvey is expected to stall over Houston for the next two days. Local officials have been criticized for not mandating an evacuation. Sunday morning, Houston Mayor Sylvester Turner told Good Morning America that, “Quite frankly, leaving your homes, getting on the streets, you’ll be putting yourself in more danger and not making yourself safer. And so, we’re just asking people to hunker down.”

But that might not be an option as the water level rises. Now the Mayor’s office is alerting people not to take shelter in attics — unless they have a way out.

Impossible to properly convey the scale of this event. The full extent won't be realized for days. Buffalo Bayou cresting 14' above record. pic.twitter.com/9PjjPLi5aC

Houston officials know that the city’s drainage infrastrastructure is inadequate and needs an overhaul. In December, the Houston Chronicle reported that the city uses once in 100-year flood levels to build drainage — in the 1980s, the city used a three-year benchmark. “Once-in-a-century storms have hit eight times in the last 27 years,” the report said. Now, the city is seeing the second storm in as many decades to break the once-in-500-years threshold.